WarmSpgs@aol.com wrote:
> Greetings, Peter and the group.
>> That is a worthwhile observation about the difference between earth rods and
> radial wires. License-free 1750-meter beacon operators in the US generally
> follow the practice of mediumwave broadcasters, utilizing buried radial wires
> to minimize loss when displacement current returns through soil resistance.
> However, experiments with radials at or above the surface have shown promise
> for reducing losses further, especially where soil conductivity is poor.
>> One of the complications in this scheme is interaction between surface or
> elevated radial wires and the ground itself, as reflected in your
> measurements. Broadcasters have used counterpoise systems in the past, and
> are once again experimenting with elevated radials; thus, a few techniques for
> coping with this problem have been developed.
>> One approach is to make the antenna and its radial system substantially self-
> contained. The regulations which apply to our LowFER experimenters pretty
> much preclude any transmission line. Therefore, when Todd, WD4NGG, tested an
> LF elevated-radial system some years ago, his entire transmitter had to be
> located at the antenna feed point. He isolated the power and control lines
> with RF chokes, I believe, but also took the step of tuning the radial system
> with a variable inductor between the transmitter case and the tie point for
> the radials. In this way, he was able to balance out any interaction. After
> tuning the radial system, making or interrupting a connection with the
> physical earth had no effect on transmitter tuning or radiation efficiency.
>> Another approach...and one which is more physically realizable in some
> circumstances...is to keep the earth, the counterpoise, and the radiating
> element each at opposite polarity to the others. The objective is to balance
> the potential of the physical earth between that of the antenna and the
> counterpoise. Hence, no displacement current needs to flow in the lossy soil.
> I apologize for not being very good with "ASCII art," but if you view the
> following with a monospaced font, perhaps it will make sense:
>> ___
> \ /
> |
> |
> )
> (variometer) )
> )
> |
> |
> )
> )--- (RF feed)
> ___(I)__)_______________
> | )
> | )
> ----- )
> (earth) --- |
> |
> =================
> (elevated radials or counterpoise)
>> Connection to the conventional earth system is made by a tap somewhere near
> the center of the balancing inductor. The RF ammeter symbolized by (I) will
> indicate minimum current when the correct tap point is found. If RF energy is
> fed to the antenna system by coaxial cable, the shield of the coax is tied to
> the same point as the earth connection.
>> Perhaps these thoughts will help in planning your elevated system. One thing
> that has been noted during computer modeling (and confirmed experimentally) is
> the desirability of elevating the radials enough to get away from interaction
> with the soil (i.e., coupling of magnetic fields arising from current in the
> radial wires). At mediumwave frequencies, a height of greater than 3 to 5
> meters is deemed best. At lower frequencies, still greater heights would be
> preferred. One may not be entirely able to achieve the desired elevation at
> LF, but I believe you will probably obtain worthwhile results within practical
> constraints.
>> 73,
> John Davis KD4IDY
>> (G3PLX wrote:) >I have been doing measurements on a 36ft vertical antenna,
> >interesting result, namely that the losses are LOWER when I use a
> >set of radial wires over the ground that when I use earth rods. If I
> >connect the earth rods in parallel with the radials, the losses get
> >WORSE and this is still the case if I bond the whole system to the
> >house safety earth. I think this can be explained by the radials being
> >a higher impedance than the ground rods but this impedance is a
> >purer capacitance. (etc) >>